Well, Human sewage is one way to cause water pollution.Sewage is dumped into water. When these wastes break down more and more, it gets rid of oxygen from the water. Which causes animals and plants to die!
Acids from companies can fall as rain, (which is called acid rain.) making waters sour/acidic.
Oil spills are another thing that causes water pollution. Oil spills "upset" water animals. Many animals could breath the oil, or even eat the oil!